Fracture Behavior of AISI M-2 High Speed Tool Steel
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EXPERIMENTAL PROCEDURE Materials and Heat Treatment The M-2 high speed steels obtained from commercial sources were melted by several methods. These included air-melted (AIR), electroslag remelted (ESR) and argon-oxygen decarburized (AOD) processes. They were received in the annealed condition in two different forms: 1) AIR material in 19.0 mm x 44.5 mm section bar stock, and 2) ESR and AOD materials in 54.0 mm diam round stock. Their respective chemical analyses are listed in Table I. Oversized compact tension and Charpy V-notch specimens were preheated in a salt bath at 815 ~ for 15 min and austenitized in salt at a temperature range of 1090 to 1230 ~ Hold the specimens at hardening temperature for 5 min without further soaking, then step quenching in salt at 590 ~ until equalized (about 5 min), and then air cooled to ambient temperature. Various authors have reported 5-7.9 the enhancement of fracture toughness in alloy steels by means of high austenitizing temperatures. In order to take advantage of the beneficial effects of high temperature austenitizing yet keep the grain size under control, a dual austenitizing process was investigated. In this process, specimens were first given a preheat austenitizing at the high end of its austenitizing range, then followed by quenching and reaustenitizing at the intended temperatures. Selected tempering of the specimens were then perTable I. Chemical Composition (Pct) of M-2 High Speed Steel Material
S. C. LEE is Assistant Professor of Mechanical Engineering, Iowa State University, Ames, IA 50011, and F. J. W O R Z A L A is Professor of Metallurgical Engineering, University of Wisconsin, Madison, WI 53706. Manuscript submitted September 15, 1980. METALLURGICAL TRANSACTIONS A
AIR ESR AOD
C
Mn
Si
Cr
Mo
W
0.83 0.86 0.78
0.38 0.35 0.44
0.36 0.29 0.36
4.83 4.04 3.90
4.94 5.01 4.90
6.58 6.49 6.60
V
P
S
2.56 * * 1.80 0.016 0.004 1.87 0.010 0.004
9 Not analyzed.
ISSN 0360-2133/81/08t 1-1477500.75/0 9 1981 A M E R I C A N SOCIETY FOR M E T A L S A N D T H E M E T A L L U R G I C A L SOCIETY OF A I M E
V O L U M E 12A, A U G U S T 1981--1477
formed in an air furnace at temperatures around the secondary hardening peak of the steel, i.e., 510 to 620 ~ for 2 + 2 hours. Detailed heat treatment schedules are shown in Table II and Table III. After heat treatment, specimens were checked for quench cracks by a 10 times stereomicroscope before final grinding to size. Test Specimens and Test Procedures Fracture toughness tests were performed by using compact tension (CT) specimens in accordance With ASTM Standard E399-74. The thickness of these specimens was 16.5 mm with W -- 33.0 mm. Specimens were made in two orientations. In ESR and AOD materials, the machined slot was transverse to the axis but in a plane parallel to the bar axis. In AIR material, it was perpendicular to the bar axis. However, there were no significant differences in the results of the two, as will be discussed later. Due to the inherent brittle nature of the high speed steel, a vertical drop-weigh
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